i think i get gypsy's point. having (conceiving) babies from your close relative does really cause problems. something goes wrong with the genes or stuff..
.. thats why it was considered taboo ( i believe i read it somewhere before )

but royal families, rich people.. they do these stuffs cuz they dont wanna distribute their fortune.

presently, i think this issue is now also a question of morality, for some. but really, it is taboo cuz of the effects (defects) that their offspring may possibly encounter.